Medical implant materials, in particular orthopedic implant materials, must combine high strength, corrosion resistance and tissue compatibility. The longevity of the implant is of prime importance especially if the recipient of the implant is relatively young because it is desirable that the implant function for the complete lifetime of a patient. Because certain metal alloys have the required mechanical strength, corrosion resistance, and biocompatibility, they are ideal candidates for the fabrication of prostheses. These alloys include 316L stainless steel, chrome-cobalt-molybdenum alloys (Co—Cr), titanium alloys, and more recently zirconium alloys, which have proven to be among the most suitable materials for the fabrication of load-bearing and non-load bearing prostheses.
To this end, oxidized zirconium orthopedic implants have been shown to reduce polyethylene wear significantly. The use of diffusion-hardened oxide surfaces such as oxidized zirconium in orthopedic applications was first demonstrated by Davidson in U.S. Pat. No. 5,037,438. Previous attempts have been made to produce oxidized zirconium coatings on zirconium parts for the purpose of increasing their abrasion resistance. One such process is disclosed in U.S. Pat. No. 3,615,885 to Watson which discloses a procedure for developing thick (up to 0.23 mm) oxide layers on Zircaloy 2 and Zircaloy 4. However, this procedure results in significant dimensional changes especially for parts having a thickness below about 5 mm, and the oxide film produced does not exhibit especially high abrasion resistance. U.S. Pat. No. 2,987,352 to Watson discloses a method of producing a blue-black oxide coating on zirconium alloy parts to increase abrasion resistance. Both U.S. Pat. No. 2,987,352 and U.S. Pat. No. 3,615,885 produce a zirconium oxide coating on zirconium alloy by means of air oxidation.
While medical implant devices made from biocompatible metal alloys are effective, they may lack certain desirable characteristics. For example, metal alloys have relatively poor flexibility and therefore do not tend to distribute load as evenly as would be desired. Uneven loads can result in a gradual loosening of the implant. As such loosening becomes more severe, revision or replacement becomes necessary. For this reason, it is desirable to design medical implants generally and prosthetic joints specifically in such a way as to maintain or improve their in vivo stability. In addition to the development of diffusion hardened surfaces to increase service life of medical implants by increasing their resistance to circumstances causing wear, there have been efforts to increase the useful life of medical implant by improving their fixation stability. In addition to wear, an implant may eventually fail if it loosens from the implantation site. Thus, advances in the area of fixation stability will address the other major source of implant failure and would represent a significant advance in implant service life. One way this problem has historically been addressed in the past is through the use of modified surfaces for medical implants which increase surface contact area and promote bone ingrowth and ongrowth. Another more recent technique involves the use of depositing material onto the surface of an implant, the material being the emission of a plasma spray source. This is discussed in U.S. Pat. Nos. 5,807,407 and 6,582,470, among others, which are incorporated by reference as though fully disclosed herein.
Medical implants are typically made from biocompatible metal alloys, such as titanium, zirconium, or cobalt chrome alloys. Not only are these metal alloys of sufficient strength to withstand relatively extreme loading conditions but due to their metallic nature, a metallic porous coating (one example being the alloy Ti-6Al-4V) may be secured to the substrate metal alloy by a metallic bond. Such metallic porous coatings are useful for providing initial fixation of the implant immediately after surgery but also serve to facilitate long-term stability by enhancing bone ingrowth and ongrowth. It is important, however, that the process of making the porous surface does not compromise the other properties of the medical implant. If fabrication of the porous surface requires harsh conditions such as high temperatures, the microstructure of the material comprising the implant may be compromised.
One such method that is able to form a porous surface and preserve the implant material microstructure is a modification of what is known as the hot isostatic pressing process (the “HIP process”). Hot isostatic pressing is a manufacturing process normally used to increase the density of metal and ceramic materials, often resulting in improved strength or workability. The HIP process subjects a component to both elevated temperature and isostatic gas pressure in a high pressure containment vessel. An inert gas is typically used so that the component material does not react with the gas. A commonly used inert pressurizing gas is argon, although others are used as well. Examples of other inert gas include helium, xenon, and others The chamber is heated, causing the pressure inside the vessel to increase. Many systems use associated gas pumping to achieve necessary pressure level. The pressurizing gas applies pressure to the component uniformly from all directions (hence the term “isostatic”). For processing castings, the inert gas is applied between 7,350 psi. (51 MPa) and 45,000 psi. (310 MPa). 15,000 psi is a commonly used pressure. Process soak temperatures range from 900° F. (480° C.) for aluminum castings to 2400° F. (1315° C.) for nickel based superalloys. When castings are HIP-treated, the simultaneous application of heat and pressure eliminates internal voids and microporosity through a combination of plastic deformation, creep, and diffusion bonding. Primary applications are the reduction of micro-shrinkage, the consolidation of powder metals, ceramic composites, and metal cladding. HIP processing is also used as part of a sintering (powder metallurgy) process and for fabrication of metal matrix composites.
The HIP process also provides a method for producing components from diverse powdered materials, including metals and ceramics. During such manufacturing processes, a powder mixture of several elements is placed in a container, typically a steel can. The container is subjected to elevated temperature and a very high vacuum to remove air and moisture from the powder. The container is then sealed, and the HIP process is applied to the sealed container. The application of high inert gas pressures and elevated temperatures results in consolidation of the powder and the removal of internal voids. The result is a clean homogeneous material with a uniformly fine grain size and a near 100% density. HIP processing eliminates internal voids and creates clean, firm bonds and fine, uniform microstructures. These characteristics are not possible with welding or casting. The virtual elimination of internal voids enhances part performance and improves fatigue strength. The process also results in significantly improved non-destructive examination ratings.
In U.S. Pat. No. 5,201,766 to Georgette, a HIP process is used to form a porous matrix and a porous matrix having a uniform surface, depth and a controlled microstructure is provided. The '766 patent teaches the use of a HIP process to form a prosthetic device having a porous coating formed of the titanium alloy Ti-6Al-4V.
There remains a need to combine the unparalleled wear properties of diffusion hardened ceramic oxide surfaces with a metallic porous surfaces to enhance fixation stability while not compromising the microstructure of the material of the implant. The present invention provides one solution to that end.
All of the above-referenced U.S. patents and published U.S. patent applications are incorporated by reference as though fully described herein.